In recent years, kinds of projectors are widely used in schools, homes and business occasions in order to amplify image signals provided by an image signal source and then display on a projection screen. For the purpose of reducing the power consumption and the size of devices, a solid-state light-emitting element is employed in the illumination system of current projector to replace the conventional high intensity discharge (HID) lamp.
Generally, the illumination system of the projector should emit three primary colors of light, i.e. red light (R), green light (G) and blue light (B). As for the luminous efficiency of the solid-state light-emitting element, the luminous efficiency of the blue solid-state light-emitting element is higher than the luminous efficiency of the red solid-state light-emitting element and the luminous efficiency of the green solid-state light-emitting element. Since the blue solid-state light-emitting element has better luminous efficiency, the red light and the green light are produced by using a blue solid-state light-emitting element and a wheel containing phosphor coating to excite blue light as red light or green light. That is, in replace of the red solid-state light-emitting element and the green solid-state light-emitting element, the uses of the blue solid-state light-emitting element and the wheel containing phosphor coating may directly emit the red light or the green light. Consequently, the luminous efficiency of the whole illumination system is enhanced.
For example, the phosphor wheel of current illumination system transforms and excites the blue light laser emitted by a blue solid-state light-emitting element into a red light with a red phosphor agent and a red light filter. FIG. 1 schematically illustrates the configuration of a red phosphor agent segment of a conventional phosphor wheel. The conventional phosphor wheel 1 of an illumination system includes a red phosphor agent segment 11, which comprises a glass 111, a red phosphor agent 112 and a red light filter 113. When emitted from the illumination system by a blue solid-state light-emitting element, the blue light 2 is transmitted through the glass 111 of the red phosphor agent segment 11 of the phosphor wheel 1 and projected on the red phosphor agent 112, which is coated on the glass 111, such that a red light 3 is emitted from the red phosphor agent 112 and transmitted to the red light filter 113 with the rest blue light 2. Simultaneously, the red light 3 is transmitted through the red light filter 113 and the rest blue light 2 is stop because of the characteristic of the red light filter 113.
However, the poor conversion efficiency of the red phosphor agent causes lower brightness of the red light output of the illumination system. In some conventional illumination systems, even though the color performance of the projector can be promoted by increasing the size of the red phosphor agent segment of the phosphor wheel, the brightness of the white light output of the whole illumination system will be too low to perform as a normal projector. At the same time, the overall fabricating cost of the projector will be increased due to the higher price of the red phosphor agent.
Moreover, when the driving current of the blue light laser of the above-mentioned illumination system is larger, the luminance and the illuminance of the red light will decay. FIG. 2 schematically illustrates the corresponding graph of the driving current of the blue light laser of the conventional illumination system versus the illuminance of the red light. As shown in FIG. 2, when the driving current of the blue light laser rises up (e.g. more than 1.3 (A)), the illuminance of the red light decreases continuously. In other words, when the illuminance of the red light reaches a peak value, no matter how the driving current increases, the illuminance of the red light will be no longer increasing. Under this circumstance, the luminance and the illuminance will be too low so that the whole brightness cannot be integrated effectively, and further affects the sharpness and the contrast of color.
There is a need of providing an improved phosphor wheel and an illumination system with the same to obviate the drawbacks encountered from the prior art.